The window glass in buildings is generally a major point of heat entry and exit. For example, the proportion of heat lost through windows when heating a building during the winter can reach about 48%, while the proportion of heat that enters through windows when cooling during the summer can be as high as about 71%. The same phenomenon applies to automobiles, in which window glass is also a major point of heat entry and exit. In automobiles, the ratio of window glass to interior space is higher than that in buildings, leaving little room for persons inside to sun's rays, which means that the interior of an automobile located in a hot weather environment can reach an extremely high temperature.
In measurements taken in a summer environment in Japan, the air temperature inside a parked automobile has been found to reach approximately 70° C. With respect to the temperature of the interior parts of an automobile, the top of the instrument panel may rise to nearly 100° C., while the ceiling may rise to nearly 70° C. It goes without saying that riding in an automobile under such conditions is uncomfortable. Also, since the temperature of interior parts does not go down quickly even when the interior is ventilated or air-conditioned, passengers continue to be radiated with radiant heat for a long time, and this greatly diminishes the level of comfort within the vehicle.
Light control glass capable of controlling the transfer of light and heat has been developed as a technology for solving these problems. There are several types of light control systems used in light control glass. Examples of light control devices include: 1) electrochromic elements featuring a material whose optical transmissivity reversibly changes upon application of current or voltage, 2) thermochromic elements featuring a material whose transmissivity varies with temperature, and 3) gas chromic elements featuring a material whose transmissivity is varied by controlling an atmosphere gas.
Of these, electrochromic elements are able to electrically control whether or not light and heat are transmitted. Accordingly, electrochromic elements allow the transmission of light and heat to be set as the user desires, and are extremely suitable as light control materials applied to building and vehicle glass. Furthermore, since these elements maintain the same optical characteristics when no current or voltage is being applied, less energy is required to maintain a constant state.
Although some configurations of electrochromic elements are liquids, in such cases leakage of the liquid must be prevented. Since buildings and vehicles are generally used for long periods of time, although technologically feasible, preventing the leakage of liquid for an extended period of time leads to higher costs. Accordingly, all of the materials that make up an electrochromic element suitable for building and automobile glass are preferably a solid such as tungsten oxide.
Tungsten oxide and other known electrochromic elements are based on the principle of controlling light by absorbing it with a light control material. Specifically, these elements suppress the advance of heat in the form of light into an interior by absorbing the light. However, when a light control material that works by this light control principle is used, a problem is that the light control material retains heat as a result of absorbing the light, and this heat is radiated back into the interior and ends up working its way into the light control glass.
A technique for solving this problem has been proposed, in which light is controlled by reflecting the light instead of absorbing it. That is, the entrance of heat into an interior caused by thermal absorption by a light control material can be prevented by using a reflection light control material which reversibly changes between a mirror state and a transparent state.
As an example of a reflection light control electrochromic elements having this characteristic, an electrochromic element having a reflection light control layers composed of an alloy of a rare earth metal and magnesium and a hydride thereof, a proton-conductive, transparent, oxidation protective layer, an anhydrous solid electrolyte layer, and an ion storage layer has been disclosed in a prior publication (see Patent Document 1).
With this element, the reflection light control layer has a function of controlling the reflectivity of the electrochromic element, and its reflectivity changes through the transfer of protons. The oxidation protective layer is composed, for example, of a compound having proton conductivity, such as niobium oxide, vanadium oxide, tantalum oxide, and other oxides, and magnesium fluoride, lead fluoride, and other fluorides, and prevents oxidation of the reflection light control layer.
The ion storage layer accumulates protons used to control reflectivity. When voltage is applied to light control glass having the above-mentioned element, protons move from the ion storage layer into the reflection light control layer via the solid electrolyte and the oxidation protective layer, which changes the reflectivity of the reflection light control layer. When voltage is applied in the opposite direction, protons are released from the reflection light control layer, and the reflectivity of the reflection light control layer returns to its original level. However, since expensive rare earth metals are used for the reflection light control layer with this element, application to a large surface area is difficult from the standpoint of cost.
As an example of another reflection light control device that uses an inexpensive and more practical material for the reflection light control layer, an element has been proposed in which Mg2Ni is laminated as the reflection light control layer, while palladium or platinum is laminated as a catalyst layer (see Patent Document 2). However, this type of material was not at all practical because of its low transmissivity when the element is transparent. A magnesium/nickel alloy thin film developed by some of the inventors of the present invention (see Patent Document 3) is of the gas chromic type and makes use of hydrogen gas, having a visible light transmissivity thereof of about 50%, which is considerably higher than the level of 20% reported for Mg2Ni in the past, and is close to being practical.
However, with an all solid state type light control mirror optical switch employing a magnesium/nickel alloy thin film (see Patent Document 4), the film is thin and has a yellowish tint in its transmissive state, and is not completely colorless and transparent. Since a yellowish color is undesirable in building and vehicle glass, this poses a major obstacle to practical application.    Patent Document 1: Japanese Patent Application Laid-Open No. 2000-204862    Patent Document 2: U.S. Pat. No. 6,647,166    Patent Document 3: Japanese Patent Application Laid-Open No. 2003-335553    Patent Document 4: Japanese Patent Application Laid-Open No. 2005-274630
In this situation, and in light of the above-mentioned prior art, the inventors conducted diligent and extensive research aimed at developing an electrochromic element capable of fundamentally solving these problems, and as a result succeeded in developing an all solid state type reflection light control electrochromic element employing a magnesium/titanium alloy thin film, which led to the completion of the present invention.